On a day in early January, a man walked up to the front door of a South Bay home and knocked.

The homeowner, a captain in the Los Angeles Police Department, looked outside. Although the two had met before, the captain didn't recognize the large black man standing in his walkway and waited behind the closed door until he left.

Weeks passed, and the 23-year veteran of the LAPD thought nothing more of the encounter.

About a month later, the face he had seen outside his home was everywhere — staring out from the front pages of newspapers, TV screens and wanted signs on billboards throughout the city.

Police allege that by then, former LAPD Officer Christopher Dorner had begun a rampage of revenge against law enforcement officials whom he blamed for his 2009 firing from the force. Police say that he killed the daughter of a retired LAPD official and her fiance, and then two officers, before apparently taking his own life during a standoff with police at a Big Bear area cabin on Feb. 12.

In all, law enforcement sources said Dorner is believed to have stalked at least five LAPD officials connected to his firing and their family members in the weeks before he began killing.

The visit to the captain's home, and a sighting by a neighbor in another officer's neighborhood, adds to evidence that Dorner planned for weeks before taking action. In an online manifesto that police attributed to Dorner, the former officer named the captain as one of the dozens of LAPD officers with whom Dorner sought to settle scores.

Dorner also followed Monica Quan, the retired officer's daughter, in the days before he fatally shot her and her fiance in their car, Irvine police allege in court records.

"Dorner did a lot of homework and that homework no doubt included some surveillance," LAPD Chief Charlie Beck said Tuesday. "There are some indications he may have been at several homes. Nothing conclusive as of yet, but it would perfectly fit in with what he was trying to accomplish, which was find everybody and harm their families."

It is not clear why Dorner apparently went to the captain's house. He backed away from the door almost immediately after knocking and loitered in the walkway for a few minutes before leaving, sources told The Times. The captain has requested anonymity from The Times out of safety concerns.

In a second incident three days before Quan and her fiance were killed, a neighbor of Capt. Phil Tingirides was walking his dogs about 11 p.m. when he noticed a pickup truck circling the block, Tingirides said in an interview. The driver of the truck pulled up alongside the neighbor and chatted with him for a few minutes about his dogs and then drove off. It wasn't until several days later when the neighbor saw Dorner's image on media reports that he reported the encounter. "He said he was very articulate, very polite," Tingirides recalled. "And he was very confident it was Dorner."

Tingirides chaired the discipline board that ruled Dorner should be fired.

Beck and other police officials have been reluctant to divulge details of Dorner's apparent attempts to make contact with, or secretly track, officers in the weeks before he began killing because of lingering safety concerns. Although they have ended most of the armed security details that guarded more than 50 police officers and their families during the hunt for Dorner, officials remain worried about the possibility that someone with similar views might try an attack.

Despite those concerns, Tingirides and his wife, a sergeant in the LAPD, spoke publicly Tuesday about the fear that came with first hearing about Dorner's threats and the realization that their children could be in danger.

"I have been with the department for 33 years and I have had a number of threats," Tingirides said. "I have been shot at ... but when you get a phone call and they tell you someone is after your family and then within a very short distance of your home they've already killed someone else's daughter, it made me sick to my stomach."

For a week, Tingirides and his wife said, they struggled to balance safety concerns with the need to maintain a sense of normalcy for their children. Gymnastic lessons and other outings suddenly included coordinating with armed officers who shadowed their every move.

"We'd go in the garage and cry because we didn't want our kids to see the anguish and hurt we were feeling," Emada Tingirides said.

In an interview with The Times, Emada Tingirides recalled an encounter she had with Dorner shortly after the incident that ultimately led to his firing. As a rookie in 2007, Dorner accused his training officer of kicking a mentally ill man three times while taking him into custody. An internal investigation found Dorner's allegations were untrue and he was accused of fabricating the story.

Dorner anxiously approached Tingirides, who was a supervisor in his station at the time, seemingly unsettled by the intensity of the internal investigation that had been opened after he made the allegations against the officer, she said. "He was saying he felt bad about making the complaint against her, and was asking why he was being questioned by investigators," Tingirides recalled. "He was asking, 'Is it because I am black? Because I'm a rookie?' "